[PATCH v13 07/13] um: nommu: configure fs register on host syscall invocation

Hajime Tazaki
2025-11-08T08:06:13Z

As userspace on UML/!MMU also need to configure %fs register when it is
running to correctly access thread structure, host syscalls implemented
in os-Linux drivers may be puzzled when they are called. Thus it has to
configure %fs register via arch_prctl(SET_FS) on every host syscalls.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
---
arch/um/include/shared/os.h | 6 +++
arch/um/os-Linux/process.c | 6 +++
arch/um/os-Linux/start_up.c | 21 +++++++++
arch/x86/um/nommu/do_syscall_64.c | 37 ++++++++++++++++
arch/x86/um/nommu/syscalls_64.c | 71 +++++++++++++++++++++++++++++++
5 files changed, 141 insertions(+)

diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
index 5451f9b1f41e..0ac87507e05e 100644
--- a/arch/um/include/shared/os.h
+++ b/arch/um/include/shared/os.h
@@ -189,6 +189,7 @@ extern void check_host_supports_tls(int *supports_tls, int *tls_min);
extern void get_host_cpu_features(
void (*flags_helper_func)(char *line),
void (*cache_helper_func)(char *line));
+extern int host_has_fsgsbase;

/* mem.c */
extern int create_mem_file(unsigned long long len);
@@ -213,6 +214,11 @@ extern int os_protect_memory(void *addr, unsigned long len,
extern int os_unmap_memory(void *addr, int len);
extern int os_drop_memory(void *addr, int length);
extern int can_drop_memory(void);
+extern int os_arch_prctl(int pid, int option, unsigned long *arg);
+#ifndef CONFIG_MMU
+extern long long host_fs;
+#endif
+

void os_set_pdeathsig(void);

diff --git a/arch/um/os-Linux/process.c b/arch/um/os-Linux/process.c
index ddb5258d7720..dacf63ac33c8 100644
--- a/arch/um/os-Linux/process.c
+++ b/arch/um/os-Linux/process.c
@@ -18,6 +18,7 @@
#include <sys/prctl.h>
#include <sys/wait.h>
#include <asm/unistd.h>
+#include <sys/syscall.h> /* For SYS_xxx definitions */
#include <linux/threads.h>
#include <init.h>
#include <longjmp.h>
@@ -179,6 +180,11 @@ int __init can_drop_memory(void)
return ok;
}

+int os_arch_prctl(int pid, int option, unsigned long *arg2)
+{
+ return syscall(SYS_arch_prctl, option, arg2);
+}
+
void init_new_thread_signals(void)
{
set_handler(SIGSEGV);
diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 33e039d2c1bf..c0afe5d8b559 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -20,6 +20,8 @@
#include <sys/resource.h>
#include <asm/ldt.h>
#include <asm/unistd.h>
+#include <sys/auxv.h>
+#include <asm/hwcap2.h>
#include <init.h>
#include <os.h>
#include <smp.h>
@@ -37,6 +39,8 @@
#include <skas.h>
#include "internal.h"

+int host_has_fsgsbase;
+
static void ptrace_child(void)
{
int ret;
@@ -460,6 +464,20 @@ __uml_setup("seccomp=", uml_seccomp_config,
" This is insecure and should only be used with a trusted userspace\n\n"
);

+static void __init check_fsgsbase(void)
+{
+ unsigned long auxv = getauxval(AT_HWCAP2);
+
+ os_info("Checking FSGSBASE instructions...");
+ if (auxv & HWCAP2_FSGSBASE) {
+ host_has_fsgsbase = 1;
+ os_info("OK\n");
+ } else {
+ host_has_fsgsbase = 0;
+ os_info("disabled\n");
+ }
+}
+
void __init os_early_checks(void)
{
int pid;
@@ -488,6 +506,9 @@ void __init os_early_checks(void)
using_seccomp = 0;
check_ptrace();

+ /* probe fsgsbase instruction */
+ check_fsgsbase();
+
pid = start_ptraced_child();
if (init_pid_registers(pid))
fatal("Failed to initialize default registers");
diff --git a/arch/x86/um/nommu/do_syscall_64.c b/arch/x86/um/nommu/do_syscall_64.c
index 292d7c578622..9bc630995df9 100644
--- a/arch/x86/um/nommu/do_syscall_64.c
+++ b/arch/x86/um/nommu/do_syscall_64.c
@@ -2,10 +2,38 @@

#include <linux/kernel.h>
#include <linux/ptrace.h>
+#include <asm/fsgsbase.h>
+#include <asm/prctl.h>
#include <kern_util.h>
#include <asm/syscall.h>
#include <os.h>

+static int os_x86_arch_prctl(int pid, int option, unsigned long *arg2)
+{
+ if (!host_has_fsgsbase)
+ return os_arch_prctl(pid, option, arg2);
+
+ switch (option) {
+ case ARCH_SET_FS:
+ wrfsbase(*arg2);
+ break;
+ case ARCH_SET_GS:
+ wrgsbase(*arg2);
+ break;
+ case ARCH_GET_FS:
+ *arg2 = rdfsbase();
+ break;
+ case ARCH_GET_GS:
+ *arg2 = rdgsbase();
+ break;
+ default:
+ pr_warn("%s: unsupported option: 0x%x", __func__, option);
+ break;
+ }
+
+ return 0;
+}
+
__visible void do_syscall_64(struct pt_regs *regs)
{
int syscall;
@@ -13,6 +41,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
syscall = PT_SYSCALL_NR(regs->regs.gp);
UPT_SYSCALL_NR(&regs->regs) = syscall;

+ /* set fs register to the original host one */
+ os_x86_arch_prctl(0, ARCH_SET_FS, (void *)host_fs);
+
if (likely(syscall < NR_syscalls)) {
unsigned long ret;

@@ -29,4 +60,10 @@ __visible void do_syscall_64(struct pt_regs *regs)

/* handle tasks and signals at the end */
interrupt_end();
+
+ /* restore back fs register to userspace configured one */
+ os_x86_arch_prctl(0, ARCH_SET_FS,
+ (void *)(current->thread.regs.regs.gp[FS_BASE
+ / sizeof(unsigned long)]));
+
}
diff --git a/arch/x86/um/nommu/syscalls_64.c b/arch/x86/um/nommu/syscalls_64.c
index d56027ebc651..19d23686fc5b 100644
--- a/arch/x86/um/nommu/syscalls_64.c
+++ b/arch/x86/um/nommu/syscalls_64.c
@@ -13,8 +13,70 @@
#include <asm/prctl.h> /* XXX This should get the constants from libc */
#include <registers.h>
#include <os.h>
+#include <asm/thread_info.h>
+#include <asm/mman.h>
#include "syscalls.h"

+/*
+ * The guest libc can change FS, which confuses the host libc.
+ * In fact, changing FS directly is not supported (check
+ * man arch_prctl). So, whenever we make a host syscall,
+ * we should be changing FS to the original FS (not the
+ * one set by the guest libc). This original FS is stored
+ * in host_fs.
+ */
+long long host_fs = -1;
+
+long arch_prctl(struct task_struct *task, int option,
+ unsigned long __user *arg2)
+{
+ long ret = -EINVAL;
+ unsigned long *ptr = arg2, tmp;
+
+ switch (option) {
+ case ARCH_SET_FS:
+ if (host_fs == -1)
+ os_arch_prctl(0, ARCH_GET_FS, (void *)&host_fs);
+ ret = 0;
+ break;
+ case ARCH_SET_GS:
+ ret = 0;
+ break;
+ case ARCH_GET_FS:
+ case ARCH_GET_GS:
+ ptr = &tmp;
+ break;
+ }
+
+ ret = os_arch_prctl(0, option, ptr);
+ if (ret)
+ return ret;
+
+ switch (option) {
+ case ARCH_SET_FS:
+ current->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)] =
+ (unsigned long) arg2;
+ break;
+ case ARCH_SET_GS:
+ current->thread.regs.regs.gp[GS_BASE / sizeof(unsigned long)] =
+ (unsigned long) arg2;
+ break;
+ case ARCH_GET_FS:
+ ret = put_user(current->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)], arg2);
+ break;
+ case ARCH_GET_GS:
+ ret = put_user(current->thread.regs.regs.gp[GS_BASE / sizeof(unsigned long)], arg2);
+ break;
+ }
+
+ return ret;
+}
+
+SYSCALL_DEFINE2(arch_prctl, int, option, unsigned long, arg2)
+{
+ return arch_prctl(current, option, (unsigned long __user *) arg2);
+}
+
void arch_set_stack_to_current(void)
{
current_top_of_stack = task_top_of_stack(current);
@@ -48,3 +110,12 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,

return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
}
+
+static int __init um_nommu_setup_hostfs(void)
+{
+ /* initialize the host_fs value at boottime */
+ os_arch_prctl(0, ARCH_GET_FS, (void *)&host_fs);
+
+ return 0;
+}
+arch_initcall(um_nommu_setup_hostfs);
--
2.43.0